Where are the default matcap files located?
You can't add matcaps in blender 2.79 or lower, as in the release notes of Blender 2.66
(March 6th, 2013) it read:

Currently a set of 24 (GPL licensed) Matcaps have been compiled into Blender.
[...]
Loading own matcap image files will be added later. That needs design and code work to get it stable and memory-friendly.


6 June 2018: This option has been added to the development version of Blender 2.8 as shown in this youtube preview by the Blender Developers